|Publication number||US5173750 A|
|Application number||US 07/475,575|
|Publication date||Dec 22, 1992|
|Filing date||Feb 6, 1990|
|Priority date||Feb 6, 1990|
|Publication number||07475575, 475575, US 5173750 A, US 5173750A, US-A-5173750, US5173750 A, US5173750A|
|Inventors||Joseph F. Laukaitis|
|Original Assignee||Eastman Kodak Company|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (2), Referenced by (16), Classifications (16), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates generally to infrared reflection densitometers for measuring the relative optical density of a developed toner image on a test patch for controlling process parameters in an electrostatographic machine.
In electrostatographic machines such as printers and copiers, control of process parameters is required to produce images having constant and predeterminable densities. Such parameters include electrical charger energization, exposure energy, development bias voltage, toner concentration, and image transfer potential.
One method of monitoring the process parameters is to measure the developed toner mass on an image receiver; and a technique for measuring the developed toner mass is to measure the reflective optical density of an exposed and developed area, called a patch, on the image receiver.
Known techniques for measuring the relative optical density of the developed toner image include the use of infrared reflectance densitometers. In U.S. Pat. Nos. 4,550,254 and No. 4,553,033, several stages of a photodiode circuit monitors and controls the light output from a light emitting diode, detects undesirable scattered background light from the image receiver, and detects the light reflected from the developed toner image. An automatic gain control circuit adjusts the output gain of the reflection densitometer, and a sample and hold circuit adjusts for different effects of aging, current leakage, or other circuit component performance characteristics.
The system described in these prior art patents forces the output of the photodiode circuit to zero reference when there is no light input. The correction for changing electronic factors is achieved by comparing the output of the photodiodes when the light emitting diodes are off to a voltage of a reference generator, and then subtracting it from the measured density value.
It is an object of the present invention to provide a reflection densitometer which does not require the calibration step, or the attending circuit complexities, of the prior art systems.
According to one aspect of the present invention, A densitometer includes a light source positioned to project light rays toward a surface whose reflectance is to be measured. A photodetector is positioned to receive light rays reflected from the surface, and has an output which is characteristic of the amount of reflected light received plus a noise component. The output of the photodetector characteristic of only the noise component is isolated in time from the output of the photodetector characteristic of reflected light received plus the noise component.
According to a preferred feature of the present invention, the output of the photodetector which is characteristic of reflected light received plus the noise component is subtracted from the output of the photodetector which is characteristic of only the noise component to obtain a densitometer output signal characteristic of only the output of the photodetector characteristic of reflected light received.
According to another preferred feature of the present invention, the light source is switched on and off so as to create alternating first and second time periods in which the output of the photodetector is characteristic of only the noise component and in which the output of the photodetector characteristic of reflected light received plus a noise component, respectively.
According to yet another feature of the present invention, the densitometer output signal is held constant during the first and second periods by a sample and hold circuit adapted to sample during the second periods and to hold the sample during the first periods.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.
In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
FIG. 1 is a vertical schematic representation of an electrostatographic machine with which the present invention is usable;
FIG. 2 is an enlarged fragmentary perspective view of a portion of the electrostatographic machine of FIG. 1;
FIG. 3 is a schematic of a densitometer circuit of the machine of FIGS. 1 and 2; and
FIGS. 4a and 4b are signal output wave form of various component of the circuit of FIG. 3.
FIG. 1 shows an electrophotographic machine 10 having an image receiver in the form of a photoconductive belt 12 which moves in a clockwise direction, as represented by an arrow 14.
An electrostatic charge is applied to belt 12 at a charging station 16. Images are projected onto belt 12 at an exposure station 18, providing a suitable interframe distance between image areas. The projected light images dissipate the electrostatic charge at the exposed areas of the photoconductive belt to form a latent electrostatic image on belt 12.
The latent electrostatic image on belt 12 is developed with toner at a developer station 20. The toner image is then subjected to radiation by a post-development erase lamp 22 to reduce the electrical stress on photoconductive belt 12 and to reduce the attraction between the toner image and belt 12.
As the toner image on belt 12 approaches a transfer station 24, a copy sheet is fed from a supply (not shown). Transfer station 24 serves as a means to effect the movement of the toner image to copy sheets (not shown) by applying a charge opposite in polarity to that of the toner image and neutralizing the charge on the copy sheet so that it easily separates from belt 12. The copy sheet bearing toner is then passed through a pair of heated fuser rollers (not shown). Mechanical and electrical cleaning of photoconductive belt 12 is effected at a cleaning station 34.
Referring to FIG. 2, photoconductive belt 12 is illustrated with a plurality of image areas or film frames 36 spaced slightly apart from each other along the longitudinal length of the belt; thus defining non-image interframe regions 38.
In order to control the electrographic process, it is known to provide one or more test patches 40 of toner in interframe regions 38. The test patches can be formed by leaving such areas charged when the other parts of the photoconductive belt outside image areas 36 are discharged, and then exposing the area to a predetermined level of irradiation. Then toner is applied to the test patches by development station 20 of FIG. 1. In this manner the density of toner on the test patches is directly related to the density of toner in image areas 36. By way of example, three toned test patches 40 are shown adjacent to each other in each interframe region 38. However, more or fewer test patches could be provided if desired. When multiple est patches for each interframe region are used for density measurement, the patches preferably are exposed to obtain different density levels of toner so that the electrographic process can be checked and controlled for various operating parameters.
Referring back to FIG. 1, as test patches 40 pass a reflection densitometer 42, light rays from a light emitting diode reflect from the surface of each test patch to an associated photodetector. A signal generated by densitometer 42 is provided to a logic and control unit (not shown) which is programmed to provide various control signals to portions of the apparatus in response to the signal received from the densitometer. For example, the control signal from the densitometer can cause the logic and control unit to regulate a number of process parameters such as the voltage applied to photoconductive belt 12 at charging station 16, the intensity or duration of exposure at station 18, the bias voltage of development station 20, and/or the concentration of toner in the developer mixture. In general, the signal from densitometer 42 can be used to control any process parameter that effects the density of the toner images on the photoconductor.
FIG. 3 is a schematic of the circuit of densitometer 42 of FIG. 1. A light emitting diode 44 is aligned with a photodetector 46 such that the photodiode collects at least a portion of the radiation from the light emitting diode which is reflected from the surface of photoconductive belt 12 of FIG. 1. Photodetectors generally have a so called dark current, and mag respond to a certain quantity of undesirable background or stray light. Therefore, the signal from photodetector 46 includes an output component which is characteristic of the amount of reflected light received, and a noise component. The present invention provides a novel way to distinguish the two.
The light emitting diode is switched on and off by an oscillator 48. The frequency of the oscillator is chosen to correspond to the response of photodetector 46, and that of a logarithmic amplifier 50 which compresses the photodetector output signal, say 100:1. As an example, oscillator 48 may have a frequency of, say, one kHz.
The signal output of logarithmic amplifier 50 is shown in FIG. 4a. Note that logarithmic amplifier 50 inverts the signal, so the top of the wave form represents the output of photodetector 46 when light emitting diode 44 is turned off (the dark level), and the bottom of the wave form represents the output of photodetector 46 when light emitting diode 44 is turned on.
The output of logarithmic amplifier 50 is inputted to the negative terminal of a difference amplifier 52, as well as to a peak detector 54. The peak detector has a relatively long time constant so as to hold the dark level voltage output of logarithmic amplifier 50, which drifts very slowly.
The output of peak detector 54 is inputted to the positive terminal of difference amplifier 52 as a reference such that the input to a sample and hold circuit 56 is an amplified difference between the dark level peak voltage and the sampled density signals. The input to the sample and hold circuit is a square wave shown in FIG. 4b, the top of which represents the sample density.
Sample and hold circuit 56 is simply a peak detector with a time constant switched by oscillator 48 so that the frequency of light emitting diode 44 is the same as that of the sample and hold circuit. Accordingly, the output signal of the sample and hold circuit represents only the radiation reflected from the photoconductive belt surface with no component of background light or dark current. This signal can be used by the machine process control.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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|U.S. Classification||356/445, 250/214.00B, 327/514|
|International Classification||G01J1/44, G01J1/18, G01N21/27, G01J1/16|
|Cooperative Classification||G01J1/16, G01N21/27, G01J2001/184, G01J2001/1694, G01J2001/4433, G01N2201/0621, G01N21/4738|
|European Classification||G01J1/16, G01N21/27|
|Feb 6, 1990||AS||Assignment|
Owner name: EASTMAN KODAK COMPANY, A CORP. OF NJ., NEW YORK
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:LAUKAITIS, JOSEPH F.;REEL/FRAME:005227/0928
Effective date: 19900130
|May 23, 1996||FPAY||Fee payment|
Year of fee payment: 4
|May 30, 2000||FPAY||Fee payment|
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|May 28, 2004||FPAY||Fee payment|
Year of fee payment: 12